Zinc acrylate is a known chemical used as a cross-linking agent for incorporating into a rubber composition to cross-link adjacent chains of rubber molecules. After cross-linking, the physical property of the rubber, such as elasticity; tensile strength and the like, is determined by the cross-linking density of a given chain length and the number of zinc ion. U.S. Pat. No. 4,561,657 disclosed a process for producing a golf ball, comprising of coating zinc acrylate powder with fatty acid or zinc salt thereof, and then incorporating the resulting powder in the rubber composition as a cross-linking agent for the production of a solid golf ball that has enhanced impact resilience, hardness and weight uniformity. Accordingly, modified zinc acrylate powder has a practical utility in the industry.
A modified zinc acrylate refers to a powder produced by coating a fatty acid or zinc salt thereof on the zinc acrylate powder in the course of its producing. Producing of zinc acrylate by reacting zinc oxide with acrylic acid is a known process per se. Nevertheless, to produce a modified zinc acrylate powder having a specific property by adjusting the formulation and/or by changing the way of adding raw materials in the course of production comprises a specific technique. Different producing process may affect the physicochemical properties such as, particle size, zinc content and acrylate content, of the resulted modified zinc acrylate powder as well as can influence the end use of such a chemical. Various prior techniques in connection with the producing of zinc acrylate powder have been proposed, such as those described in JP-A-52-154,436, JP-A-53-83,834, JP-A-60-94,434, and JP-A-02-218,639. However, there were disadvantages associated with these prior art techniques, such as, lacking an efficient drying process for removing water and organic liquid medium, time-consumption in filtration or distillation under reduced pressure, trouble in adhering stirring blades or the wall of the reactor that might result in the loss of the product and the laborsome cleansing of the reactor, or aggregation of the crude product. With respect to the aggregated product, an additional equipment is needed for grinding and sieving such that the product can pass a specified particle size of 325 mesh (&lt;44 .mu.m).
U.S. Pat. No. 5,789,616 disclosed a process comprising adding at the first an anionic surfactant, sodium dioctyl sulfosuccinate, in toluene, following with adding successively zinc oxide, fatty acid and acrylic acid, and, after completion of the reaction, drying by distillation at a reduced pressure, and pulverizing and sieving the thus produced crude product to yield a modified zinc acrylate powder having a particle size characterized in that 64% of particles&lt;44 .mu.m, 54% of particles&lt;10 .mu.m, and 41% of particles&lt;5 .mu.m, while the part that had a particle size of&gt;105 .mu.m and must be removed by sieving was estimated to be 31%. The anionic surfactant, i.e. sodium dicotyl sulfosuccinate, used in this technique to be added at the first stage of the reaction might cause the swelling of the content in the reaction mixture such that in the subsequent addition of acrylic acid, might lead to local thickening and hence difficulty for stirring, and therefore, the reaction could be completely only through slow addition of raw material and postponing the reaction time. This might be the cause that made the zinc acrylate powder thus produced coarser and the particle size distributions non-uniform. On the other hand, it had an advantage that the trouble of sticking the wall of the reactor and the drying tank in the course of drying could be avoided.